Whenever an electrode at the end of an EKG or EEG lead is applied to a patient's body, a potential electrical shock hazard to the patient is created. Electrodes are generally designed to provide a low resistance connection to the patient so as to facilitate the detection of low amplitude physiological signals from the patient's body. However, this low resistance connection also facilitates the application of hazardous voltages to the patient and the flow of current to and from the patient, thereby exposing the patient to electrical shock. Hazardous voltages and currents may arise from static discharges, from contact of the patient or of an electrode with surrounding electrical equipment, from EKG or EEG equipment failure, from ground loops, or from stray capacitance which often develops between the lead and grounded objects to which the lead is coupled.
Many voltage and current limiting devices such as isolator circuits and non-linear series circuits have been developed to protect the patient from electrical shock. Isolator circuits generally comprise considerable floating or non-grounded circuitry, including modulators and transducers or couplers of the optical, magnetic, electric field or acoustic type. However, these circuits are usually complex and costly, and because these circuits are usually incorporated into biomedical equipment located away from the patient, they do not adequately protect the patient from electrical shock such as from long-lead shunt currents that arise from stray capacitance in the lead. Also, isolator circuits do not adequately eliminate artifacts that sometimes reside in the physiological signal received from the patient. Such artifacts are often caused by large electrostatic common mode voltages arising from multiple electrodes placed on the patient's body.
Non-linear series circuits also have certain inherent disadvantages which cause them to provide inadequate shock protection to the patient. These disadvantages are described below. Most known non-linear series circuits are semiconductor circuits using Field Effect Transistors (FET), Bipolar Transistors, or Diodes.
FET circuits are unduly complex and costly and can be damaged easily when large overload voltages are applied such as electrocautery and defibrillation voltages thereby providing inadequate patient protection. Electrocautery voltages are generally about 2 kilovolts in magnitude while defibrillation voltages are generally about 8 kilovolts in magnitude.
Bipolar transistor circuits, which are often constructed having a plurality of bipolar transistors, a battery, and one or more resistors, having the same disadvantages as FET circuits mentioned above plus the added disadvantage of limited battery life.
Diode type current-limiting circuits generally comprise diode bridge circuits, hot carrier diode circuits, germanium diode circuits or silicon diode circuits. Diode bridge circuits do not adequately protect the patient from shunt currents that occur in long leads because these circuits, by reason of their spatial requirements or bulk, are generally located in the biomedical equipment itself and away from the patient. For example, these circuits often require space to accommodate both a positive and a negative voltage source and at least four connecting leads, a signal source lead, a signal output lead and two voltage source leads.
Hot carrier diode circuits provide decreased shock protection as temperature increases. For example, the conductivity of these circuits increases by a factor of 14 (i.e., increases to fourteen times its original value) for each 25.degree. C increase in temperature. Furthermore, hot carrier diode circuits inadequately limit the amount of current flowing to a patient. Typically, these circuits permit as much as 5 milliamps per lead, or 60 milliamps via a typical configuration of twelve leads, to pass to the patient.
Germanium diode circuits often have a low breakdown voltage that is inadequate to protect the patient when line voltages of 220 volts or higher are applied. Also, because the reverse leakage current of germanium diodes is relatively high, typically from one to twenty microamperes at room temperature, and doubles every 8.degree. C above room temperature, the shock protection afforded to patients by this type of circuit decreases by 50 percent each time the temperature rises 8.degree. C. The cause of a rise in temperature may be, for example, the hot lights or other heat sources in an operating room near to the circuit.
Silicon diode circuits have a high series resistance and low conductivity due to their small reverse leakage current. Because a patient's physiological signals are typically low amplitude signals requiring for conduction a device with high conductivity, the high-resistance low-conductivity characteristic of silicon diode circuits makes these circuits unsuited to conducting these signals.